They just need to position as an alternative to the CL and be able to generate rights sales which rival the CL- which would mean more money flows to the individual clubs.
Start with top 4 Spanish, 4 Italian, 3 Portuguese and 3 from somewhere else all invited with the prize money being more than the CL.
Eventually the other countries will want to join as always happens in these types of things.
Let the clubs choose which competition they want to join our of CL and SL, end of season.
Either the CL will have the teams finishing 4-8 in Spain or the SL will have Barcelona, Madrid and then the other top 4 teams or the teams 5-7.
But what's important is that there is an alternative that challenges UEFA monopoly.
And to be honest the British clubs will always be problematic as they live in their little non-EU GBP sanctuary where the British want to have all of the benefits of EU membership while none of the liabilities.
Want to see how British involvement would go? Just research Malaysia/Singapore and the Malaysia Cup.